1. Field of the Invention
The present invention relates generally to digital signal modulators.
2. Description of the Related Art
Digital quadrature modulators generally connect first and second digital multipliers to a digital summer. A digital sine sequence that represents an analog sine signal typically drives one of the multipliers and a digital cosine sequence that represents an analog cosine signal drives the other multiplier. When the first and second multipliers respectively receive first and second digital input sequences (which may, for example, be symbols coded to represent bits of input data streams), the output of the summer is an output quadrature-modulated sequence that represents an analog signal which carries the first and second input sequences on carriers (sine and cosine) that have a quadrature relationship. This relationship can be altered to a different quadrature relationship by interchanging the sine and cosine sequences or by interchanging the first and second input sequences.
In a different modulator arrangement, the first input sequence and a selected one of the sine and cosine sequences can be applied to a summer to thereby alter the phase of the selected sequence. The phase-altered sequence and the second input sequence can then be subsequently applied to a multiplier to thereby generate a polar-modulated sequence that represents an analog signal which also carries the first and second input sequences in a quadrature relationship.
Although the modulations of quadrature modulators and polar modulators can be shown to be equivalent, one or the other may be more compatible with a particular system design. If that design changes, however, it may require the substitution of a different modulator structure.